On the theory of diamagnetism in granular superconductors

We study a highly disordered network of superconducting granules linked by weak Josephson junctions in magnetic field and develop a mean field theory for this problem. The diamagnetic response to a slow {\it variations} of magnetic field is found to be analogous to the response of a type-II superconductor with extremely strong pinning. We calculate an effective penetration depth $\lambda_g$ and critical current $j_c$ and find that both $\lambda_g^{-1}$ and $j_c$ are non-zero but are strongly suppressed by frustration.Comment: REVTEX, 12 pages, two Postscript figure

Towards a Learning-Aware Application Guided by Hierarchical Classification of Learner Profiles

Learner profiling is a methodology that draws a parallel from user profiling. Implicit feedback is often used in recommender systems to create and adapt user profiles. In this work the implicit feedback is based on the learner's answering behaviour in the Android application UnlockYourBrain, which poses different basic mathematical questions to the learners. We introduce an analytical approach to model the learners' profile according to the learner's answering behaviour. Furthermore, similar learner's profiles are grouped together to construct a learning behaviour cluster. The choice of hierarchical clustering as a means of classification of learners' profiles derives from the observations of learners behaviour. This in turn reflects the similarities and subtle differences of learner behaviour, which are further analysed in more detail. Building awareness about the learner's behaviour is the first and necessary step for future learning-aware applications

Editorial: E-Learning-Strategien für die Hochschullehre

27.04.2015 | Sabine Seufert (St. Gallen), Martin Ebner, Michael Kopp (Graz) & Bettina Schlass (Amsterdam

Precursors of Cytochrome Oxidase in Cytochrome-Oxidase-Deficient Cells of Neurospora crassa

Three different cell types of Neurospora crassa deficient in cytochrome oxidase were studied: the nuclear mutant cni-1, the cytoplasmic mutant mi-1 and copper-depleted wild-type cells. * 1. The enzyme-deficient cells have retained a functioning mitochondrial protein synthesis. It accounted for 12–16% of the total protein synthesis of the cell. However, the analysis of mitochondrial translation products by gel electrophoresis revealed that different amounts of individual membrane proteins were synthesized. Especially mutant cni-1 produced large amounts of a small molecular weight translation product, which is barely detectable in wild-type. * 2. Mitochondrial preparations of cytochrome-oxidase-deficient cells were examined for precursors of cytochrome oxidase. The presence of polypeptide components of cytochrome oxidase in the mitochondria was established with specific antibodies. On the other hand, no significant amounts of heme a could be extracted. * 3. Radioactively labelled components of cytochrome oxidase were isolated by immunoprecipitation and analysed by gel electrophoresis. All three cell types contained the enzyme components 4–7, which are translated on cytoplasmic ribosomes. The mitochondrially synthesized components 1–3 were present in mi-1 mutant and in copper-depleted wild-type cells. In contrast, components 2 and 3 were not detectable in the nuclear mutant cni-1. Both relative and absolute amounts of these polypeptides in the enzyme-deficient cells were quite different from those in wild-type cells. * 4. The components of cytochrome oxidase found in the enzyme-deficient cells were tightly associated with the mitochondrial membranes. * 5. Processes, which affect and may control the production of enzyme precursors or their assembly to a functional cytochrome oxidase are discussed

Nonlinear Seebeck Effect in a Model Granular Superconductor

The change of the Josephson supercurrent density of a weakly-connected granular superconductor in response to externally applied arbitrary thermal gradient dT/dx (nonlinear Seebeck effect) is considered within a model of 3D Josephson junction arrays. For dT/dx>(dT/dx)_c, where (dT/dx)_c is estimated to be of the order of 10^4 K/m for YBCO ceramics with an average grain's size of 10 microns, the weak-links-dominated thermopower S (Seebeck coefficient) is predicted to become strongly dT/dx-dependent.Comment: REVTEX, no figure

Spin glass behavior of frustrated 2-D Penrose lattice in the classical planar model

Via extensive Monte Carlo studies we show that the frustrated XY Hamiltonian on a 2-D Penrose lattice admits of a spin glass phase at low temperature. Studies of the Edwards-Anderson order parameter, spin glass susceptibility, and local (linear) susceptibility point unequivocally to a paramagnetic to spin glass transition as the temperature is lowered. Specific heat shows a rounded peak at a temperature above the spin glass transition temperature, as is commonly observed in spin glasses. Our results strongly suggest that the critical point exponents are the same as obtained by Bhatt and Young in the ${\pm}J$ Ising model on a square lattice. However, unlike in the latter case, the critical temperature is clearly finite (nonzero). The results imply that a quasiperiodic 2-D array of superconducting grains in a suitably chosen transverse magnetic field should behave as a superconducting glass at low temperature.Comment: RevTex, 4 pages Including 4 figures. To appear in the June 1 1996 issue of Phys. Rev. B (Rapid Communications). Revised/replaced edition contains an erratum at the end of the paper, also to appear in Phys. Rev.

Indication of Superconductivity at 35 K in Graphite-Sulfur Composites

We report magnetization measurements performed on graphite--sulfur composites which demonstrate a clear superconducting behavior below the critical temperature T$_{c0}$ = 35 K. The Meissner-Ochsenfeld effect, screening supercurrents, and magnetization hysteresis loops characteristic of type-II superconductors were measured. The results indicate that the superconductivity occurs in a small sample fraction, possibly related to the sample surface.Comment: published versio

The test of exponentiality based on the mean residual life function revisited

We revisit the family of goodness-of-fit tests for exponentiality based on the mean residual life time proposed by Baringhaus & Henze (2008). We motivate the test statistic by a characterisation of Shanbhag (1970) and provide an alternative representation, which leads to simple and short proofs for the known theory and an easy to access covariance structure of the limiting Gaussian process under the null hypothesis. Explicit formulas for the eigenvalues and eigenfunctions of the operator associated with the limit covariance are derived using results on weighted Brownian bridges. In addition we provide further asymptotic theory under fixed alternatives and derive approximate Bahadur efficiencies, which provide an insight into the choice of the tuning parameter with regard to the power performance of the tests